1. Field of the Invention
The present invention relates to apparatus and methods for extending depth of field in image projection systems.
2. Description of the Prior Art
In projections systems, such as view graph projectors and lithographic projectors, the depth of focus of the projected image is often too small for the application. For example, in a view graph machine, it is often impossible to bring both the top and the bottom of the image in focus at the same time. Reducing the diameter of the lens increases the depth of field of the projected image, but results in a loss of light in the image.
In lithographic systems the depth of field problem is much more serious. Lithography utilizes a light source to project light (e.g. from a mercury arc lamp or a laser) onto a mask carrying the pattern to be transferred. The portion of light which is not blocked by the mask is focussed onto a thin film of photosensitive material (called resist) which has been deposited on the surface of the wafer. The pattern is developed on the resist.
In conventional lithographic systems, the depth of field of the focussing lens is so small that the wafer must be level and smooth to within a small fraction of a micrometer. This is very difficult to achieve. If the wafer thickness varies slightly or if the wafer is not perfectly seated, portions of the pattern image are not in focus.
One method which has been used to increase the depth of field of the pattern image is to decrease the diameter of the lens aperture, but this decreases the resolution of the image, as well as resulting in a loss of light. Resolution especially is of major importance in lithography.
In patent application Ser. No. 08/823,894, entitled "Extended Depth of Field Optical Systems" (incorporated herein by reference), methods and apparatus were disclosed which increase the depth of field of an image by providing an Extended Depth of Field (EDF) mask in the imaging system combined with post processing of the captured image. The EDF mask modifies the optical transfer function of the imaging system to be insensitive to the distance between the object and the imaging system. The post processing reverses the mask modifications. This method is not useful in projection systems, because it is not practical to provide post processing of the projected image.
A need remains in the art for apparatus and methods which extend depth of field in image projection systems.